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United States Patent |
5,302,773
|
Vrieland
,   et al.
|
April 12, 1994
|
Process of oxidizing aliphatic hydrocarbons employing a molybdate
catalyst encapsulated in a hard, glassy silica matrix
Abstract
A process for preparing olefins and diolefins in high productivity which
involves contacting an aliphatic hydrocarbon, such as butane, with a
heterogeneous catalyst composition containing reactive oxygen under
reaction conditions sufficient to produce a more highly unsaturated
aliphatic hydrocarbon, such as 1,3-butadiene. The catalyst composition
contains a glassy silica matrix of specified surface area and
macro-porosity into which are encapsulated domains of a catalyst component
containing oxides of magnesium and molybdenum. The catalyst has high crush
strength and is useful in transport reactors.
Inventors:
|
Vrieland; G. Edwin (Midland, MI);
Doktycz; Stephen J. (Midland, MI);
Khazai; Bijan (Midland, MI)
|
Assignee:
|
The Dow Chemical Company (Midland, MI)
|
Appl. No.:
|
797882 |
Filed:
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November 26, 1991 |
Current U.S. Class: |
585/624; 502/254; 502/255; 502/306; 585/630; 585/631; 585/658; 585/663 |
Intern'l Class: |
C07C 005/09; B01J 021/08 |
Field of Search: |
502/255,254,306
585/624,630,631,658,663
|
References Cited
U.S. Patent Documents
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| |
2426118 | Aug., 1947 | Parker, Jr. et al.
| |
2941958 | Jun., 1960 | Connor, Jr. et al.
| |
3119111 | Jan., 1964 | McDonald et al.
| |
3146210 | Aug., 1964 | Baldwin.
| |
3180903 | Apr., 1965 | Lindquist et al.
| |
3488402 | Jan., 1970 | Michaels et al.
| |
3598759 | Aug., 1971 | Bertolacini.
| |
3758418 | Aug., 1973 | Leonard, Jr. et al.
| |
3862256 | Jan., 1975 | Isailingold et al.
| |
3928238 | Dec., 1975 | Koberstein et al.
| |
3959182 | May., 1976 | Izawa et al.
| |
4035417 | Jul., 1977 | Izawa et al.
| |
4059658 | Nov., 1977 | Shoup et al.
| |
4112032 | Sep., 1978 | Blaszyk et al.
| |
4170570 | Oct., 1979 | Zagata et al.
| |
4229604 | Oct., 1980 | Tmenov et al.
| |
4276196 | Jun., 1981 | Dalton et al.
| |
4280929 | Jul., 1981 | Shaw et al.
| |
4388223 | Jun., 1983 | Ferlazzo et al.
| |
4447558 | May., 1984 | Sasaki et al.
| |
4453006 | Jun., 1984 | Shaw et al.
| |
4559320 | Dec., 1985 | Reusser.
| |
4764498 | Aug., 1988 | Wissner et al.
| |
4895821 | Jan., 1990 | Kainer et al.
| |
4902442 | Feb., 1990 | Garces.
| |
4914073 | Apr., 1990 | Grimm et al.
| |
4966877 | Oct., 1990 | Langerbeins et al.
| |
4973791 | Nov., 1990 | Vrieland et al. | 585/624.
|
Foreign Patent Documents |
0225062 | Jun., 1987 | EP.
| |
Other References
Chemical Abstracts 88-136328/20 (1986).
Chemical Abstracts 104:131946b (1986).
Chemical Abstracts 86-316078/48 (1985).
Chemical Abstracts 85-232465/38 (1984).
Chemical Abstracts 86-096610/15 (1984).
Derwent 91463R-AE (1970).
|
Primary Examiner: Garvin; Patrick P.
Assistant Examiner: Irzinski; E. D.
Attorney, Agent or Firm: Zuckerman; Marie F.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of application Ser. No. 505,751,
filed Apr. 6, 1990, and now U.S. Pat. No. 5,146,031 which is a
continuation-in-part of application Ser. No. 383,107, filed Jul. 20, 1989,
now U.S. Pat. No. 4,973,791, issued Nov. 27, 1990.
Claims
What is claimed is:
1. A process of preparing an unsaturated aliphatic hydrocarbon comprising
contacting an aliphatic hydrocarbon having at least three carbon atoms
with a solid heterogeneous catalyst composition having reactive oxygen and
having a crush strength of at least about 0.60 lb, the catalyst
composition comprising a glassy silica matrix having a BET surface area no
greater than about 20 m.sup.2 /g and having macropores ranging in size
from about 500 .ANG. to about 4000 .ANG., the silica matrix comprising
from about 25 to about 90 weight percent of the catalyst composition and
having encapsulated therein domains of a catalyst component comprising an
oxide of magnesium and an oxide of molybdenum, the contacting occurring
under conditions such that an unsaturated aliphatic hydrocarbon is
produced in a productivity of at least about 0.15 g/g cat-hr.
2. The process of claim 1 wherein the aliphatic hydrocarbon is an alkane
represented by the general formula:
CH.sub.3 --(CH.sub.2).sub.n --CH.sub.3
wherein n is an integer from 1 to about 8.
3. The process of claim 2 wherein n is 2 and the alkane is n-butane.
4. The process of claim 1 wherein the aliphatic hydrocarbon is diluted with
a non-reactive gas.
5. The process of claim 4 wherein the hydrocarbon concentration ranges from
about 40 mole percent to about 100 mole percent.
6. The process of claim 1 wherein the catalytic component consists
essentially of an oxide of magnesium and an oxide of molybdenum.
7. The process of claim 1 wherein the oxide of magnesium and the oxide of
molybdenum are partially combined in the form of magnesium molybdate.
8. The process of claim 1 wherein the temperature is in the range from
about 400.degree. C. to about 700.degree. C.
9. The process of claim 1 wherein the aliphatic hydrocarbon partial
pressure is in the range from about subatmospheric to about 100 psig.
10. The process of claim 1 wherein the gas hourly space velocity of the
feedstream is in the range from about 100 hr.sup.-1 to about 20,000
hr.sup.-1.
11. The process of claim 1 wherein the unsaturated aliphatic hydrocarbon is
a diolefin and wherein the diolefin is represented by the general formula:
CH.sub.2 .dbd.CH--CH.dbd.CH--(CH.sub.2).sub.m --H
wherein m is an integer from 0 to about 6.
12. The process of claim 11 wherein m is 0 and the diolefin is
1,3-butadiene.
13. The process of claim 1 wherein the catalyst composition has a
productivity of at least about 0.2 g unsaturated aliphatic hydrocarbons/g
cat-hr.
14. The process of claim 1 wherein the catalyst component contains a
promoting amount of an alkali metal promoter.
15. The process of claim 14 wherein the alkali metal promoter is an alkali
metal oxide, hydroxide, carbonate, acetate, or oxalate.
16. The process of claim 14 wherein the alkali metal is cesium.
17. The process of claim 14 wherein the alkali metal is potassium.
18. The process of claim 14 wherein the concentration of the alkali metal
promoter is in the range from about 0.01 weight percent to about 5 weight
percent calculated as the alkali hydroxide and based on the combined
weights of silica, magnesium oxide and molybdenum oxide.
19. The process of claim 18 wherein the concentration of alkali metal
promoter is in the range from about 0.02 weight percent to about 2 weight
percent.
20. A process of preparing 1,3-butadiene comprising contacting n-butane
with a solid heterogeneous catalyst composition containing reactive oxygen
and having a crush strength of at least about 0.60 lb, said catalyst
comprising a glassy silica matrix having a BET surface area no greater
than about 20 m.sup.2 /g and having macropores ranging in diameter from
about 500 .ANG. to about 4000 .ANG., the silica matrix comprising from
about 25 to about 90 weight percent of the catalyst composition and having
encapsulated therein domains of a catalytic component comprising magnesia
and molybdenum oxide, the contacting occurring at a temperature in the
range from about 500.degree. C. to about 650.degree. C., and a pressure in
the range from about 1 psig to about 30 psig and under such other reaction
conditions that a mixture of products is formed containing 1,3-butadiene
in a productivity of at least about 0.10 g/g cat-hr.
21. The process of claim 20 wherein the selectivity to butadiene is at
least about 60 mole percent.
22. The process of claim 20 wherein the selectivity to butadiene is at
least about 70 mole percent.
23. The process of claim 20 wherein the productivity of butadiene is at
least about 0.2 g/g cat-hr.
24. The process of claim 20 wherein the concentration of silica in the
catalyst composition ranges from about 35 to about 50 weight percent.
25. The process of claim 20 wherein the crush strength of the catalyst
composition i3 at least about 1.00 lb.
26. A solid heterogeneous catalyst composition capable of providing a
reactive form of oxygen and having a crush strength of at least about 0.60
lb, the composition comprising a glassy silica matrix having a BET surface
area no greater than about 20 m.sup.2 /g and having macropores ranging in
diameter from about 500 .ANG. to about 4000 .ANG., the silica matrix
comprising from about 25 to about 90 weight percent of the catalyst
composition and having encapsulated therein domains of a catalytic
component comprising magnesia and molybdenum oxide.
27. The process of preparing the catalyst of claim 26 comprising: (a)
treating a source of magnesium oxide with a blocking agent, (b) adding the
treated source of magnesium oxide to an alkali metal silicate solution,
the silicate being present in a concentration sufficient to provide silica
in an amount ranging from about 25 to about 90 weight percent of the
catalyst composition, (c) polymerizing the silicate to form a composite
comprising a glassy silica matrix having a BET surface area no greater
than about 20 m.sup.2 /g and having macropores ranging from about 500
.ANG. to about 4000 .ANG. in diameter, the matrix containing domains of
the treated source of magnesium oxide, (d) ion-exchanging the composite
with an ammonium salt to reduce the concentration of alkali metal ions,
(e) drying and calcining the composite under conditions sufficient to
remove the blocking agent and sufficient to convert the source of
magnesium oxide into magnesium oxide, (f) impregnating the domains of
magnesium oxide with a source of an oxide of molybdenum, (g) calcining the
resulting impregnated composite under conditions sufficient to convert the
source of an oxide of molybdenum to an oxide of molybdenum.
28. The catalyst composition of claim 26 wherein the catalytic component
contains a promoting amount of an alkali metal promoter.
29. The catalyst composition of claim 28 wherein the concentration of the
alkali metal promoter ranges from about 0.01 weight percent to about 5
weight percent calculated as alkali metal hydroxide and based on the
combined weights of silica, magnesium oxide and molybdenum oxide.
30. The catalyst composition of claim 28 wherein the alkali metal promoter
is an alkali metal oxide, hydroxide, carbonate, acetate, or oxalate.
31. The catalyst composition of claim 28 wherein the alkali metal promoter
is an oxide or hydroxide of potassium or cesium.
32. The catalyst composition of claim 26 wherein the crush strength is at
least about 1.00 lb.
33. The process of preparing the catalyst of claim 28 comprising: (a)
treating a source of magnesium oxide with a blocking agent, (b) adding the
treated source of magnesium oxide to an alkali metal silicate solution,
the silicate being present in a concentration sufficient to provide silica
in an amount ranging from about 25 to about 90 weight percent of the
catalyst composition, (c) polymerizing the silicate to form a composite
comprising a glassy silica matrix having a BET surface area no greater
than about 20 m.sup.2 /g and having macropores ranging from about 500
.ANG. to about 4000 .ANG. in diameter, the matrix containing domains of
the treated source of magnesium oxide, (d) ion-exchanging the composite
with an ammonium salt to reduce the concentration of alkali metal ions,
(e) drying and calcining the composite under conditions sufficient to
remove the blocking agent and sufficient to convert the source of
magnesium oxide into magnesium oxide, (f) impregnating the domains of
magnesium oxide with a source of an oxide of molybdenum and a source of an
oxide of an alkali metal, (g) calcining the resulting impregnated
composite under conditions sufficient to convert the sources of an oxide
of molybdenum and oxide of alkali metal to an oxide of molybdenum and an
oxide of alkali metal.
34. The process of claim 33 wherein the polymerization of the silicate is
effected by the suspension polymerization method.
35. A process of preparing a composite material comprising a glassy silica
matrix having a BET surface area no greater than about 20 m.sup.2 /g and
having macropores ranging from about 500 .ANG. to about 4000 .ANG. in
diameter, the silica matrix having encapsulated therein domains of a metal
oxide phase, the process comprising:
(a) treating a source of the metal oxide with a blocking agent, the metal
oxide being selected from those reactive with an alkali metal silicate,
(b) adding the treated source of the metal oxide to an alkali metal
silicate solution,
(c) polymerizing the silicate to form a composite comprising a glassy
silica matrix having a BET surface area no greater than about 20 m.sup.2
/g and having macropores ranging from about 500 .ANG. to about 4000 .ANG.
in diameter, the matrix containing domains of the treated source of metal
oxide phase, and
(d) calcining the composite under conditions sufficient to remove the
blocking agent and to convert the source of metal oxide into metal oxide.
36. The process of claim 35 wherein spheroidal particles are formed by the
suspension polymerization method or by spray-drying.
37. The process of claim 35 wherein an ion-exchange procedure is conducted
after the polymerization of the silicate (Step c) and before calcination
(Step d) to reduce the concentration of alkali metal ions.
38. The process of claim 35 wherein the blocking agent is poly(vinyl
alcohol), or a polyacrylic acid or polymethacrylic acid or salt thereof.
Description
BACKGROUND OF THE INVENTION
This invention pertains to the oxidation of aliphatic hydrocarbons, such as
alkanes and monoolefins, in the presence of a molybdate catalyst to form
more highly unsaturated aliphatic hydrocarbons.
Unsaturated aliphatic hydrocarbons, such as monoolefins and diolefins, are
useful as monomers and comonomers in the preparation of polyolefin
plastics.
U.S. Pat. No. 3,119,111 discloses a process for the oxidative
dehydrogenation of a C.sub.4 to C.sub.6 alkane having a four carbon chain
to a 1,3-alkadiene. The reaction occurs in the presence of oxygen and a
catalyst containing an alkali metal molybdate, such as lithium molybdate.
It is taught that the catalyst can be employed with a carrier material,
such as powdered alumina. Disadvantageously, this process requires
potentially explosive mixtures of alkanes and oxygen. More
disadvantageously, the catalyst of this process contains a high
concentration of alkali metal which lowers catalytic activity.
U.S. Pat. No. 3,180,903 discloses a process for the dehydrogenation of
aliphatic hydrocarbons containing from two to five carbon atoms. Butanes,
for example, can be converted to butenes and butadienes. The catalyst is
taught to contain chromium oxides or molybdenum oxides supported on a
gel-type alumina. Optionally, the catalyst may contain one or more alkali
metal oxides. Disadvantageously this process is limited to a low
hydrocarbon conversion and a low ultimate yield of butadiene.
U.S. Pat. No. 3,488,402 teaches the dehydrogenation of butane to butene and
butadiene in the presence of two catalysts. The first is a dehydrogenation
catalyst containing alumina, magnesia, or combinations thereof, promoted
with an oxide of a metal of Groups IVB, VB or VIB, such as chromia,
vanadium oxide or molybdenum oxide. The second catalyst is an oxidation
catalyst comprising a Group TVA or VA vanadate, molybdate,
phosphomolybdate, tungstate or phosphotungstate. Disadvantageously, this
process comprises two steps and requires subatmospheric pressures. Even
more disadvantageously, this process produces low butadiene selectivity
and yield.
U.S. Pat. No. 3,862,256 discloses a process for the oxidative
dehydrogenation of paraffin hydrocarbons, such as butane, over a catalyst
containing oxy compounds of molybdenum and magnesium and up to 20 weight
percent silicon oxide. When butane is contacted with the catalyst, the
products include butenes and butadiene; however, the selectivity and
space-time yield of butadiene are lower than desired. In addition, the
feed contains hydrocarbon and oxygen, which is not desirable for safety
reasons. Finally, the magnesium oxide support does not possess the
strength and attrition resistance needed for fluid bed or transport
reactors.
U.S. Pat. No. 4,229,604 discloses a process for the oxidative
dehydrogenation of a paraffin, such as butane, to unsaturated
hydrocarbons, such as butenes and butadiene. The catalyst contains
molybdenum and magnesium oxides which may be impregnated into a carrier
consisting of granulated porous crystalline silica modified with alkali
carbonate. The catalyst may comprise up to 20 percent by weight carrier.
It is taught that during carrier preparation silicates of the alkali
metals are formed. It is further taught that on the surface of the
catalyst there exists an active magnesium molybdate. Disadvantageously,
the catalyst produces a selectivity and space-time yield of butadiene
which are too low for industrial use.
U.S. Pat. No. 4,388,223 discloses the oxidizing dehydrogenation of butene-1
to butadiene. The catalyst comprises (a) a crystalline phase (I)
consisting of one or more molybdates belonging to the monoclinic system,
chosen from ferric, aluminum, cerium, and chromium molybdates, (b) a
crystalline phase (II) consisting of one or more molybdates belonging to
the monoclinic system, including magnesium molybdate, and (c) one or more
promoter elements including vanadium. It is also taught that the catalyst
may comprise alkaline elements such as potassium, lithium, cesium and
magnesium and/or acidic elements, such as phosphorus and silicon. In one
embodiment the catalytic metallic salts are used to impregnate
microspheroidal silica. In another embodiment a soluble colloidal silicate
is added to the solution of catalytic metallic salts, and the mixture is
spray dried and thermally activated to obtain the catalyst. This process
co-feeds hydrocarbon and oxygen, which is undesirable for safety reasons.
Moreover, the catalyst does not have the strength and attrition resistance
required for fluid-bed or transport reactors.
While the oxidation of aliphatic hydrocarbons is well researched in the
prior art, the selectivity and space-time yield to particular unsaturated
hydrocarbons, such as diolefins, fall short of those which are desired for
commercial exploitation. Moreover, the catalysts employed in the prior art
do not possess the strength and attrition resistance required for use in
industrial fluid bed or transport reactors. Accordingly, it would be
desirable to have a selective, direct oxidation of an aliphatic
hydrocarbon, such as an alkane or monoolefin, to the corresponding
unsaturated aliphatic hydrocarbons, specifically the diolefin. It would be
more desirable if such an oxidation produced a high selectivity and high
productivity of the diolefin and other olefins, and correspondingly low
selectivities to deep oxidation products, such as carbon dioxide. Finally,
it would be most desirable if the above-identified process could be
accomplished with a catalyst having a high strength and attrition
resistance so as to be useful in a commercial scale fluid bed or transport
reactor.
SUMMARY OF THE INVENTION
In one aspect, this invention is a process of preparing an unsaturated
aliphatic hydrocarbon comprising contacting an aliphatic hydrocarbon
having at least three carbon atoms with a catalyst of this invention,
described hereinafter. Under the reaction conditions of the process of
this invention more unsaturated aliphatic hydrocarbons, such as diolefins,
are formed in a productivity of at least about 0.15 gram per gram catalyst
per hour (g/g cat-hr).
Advantageously, aliphatic hydrocarbons can be oxidized directly to more
highly unsaturated aliphatic hydrocarbons by the process of this
invention. Surprisingly, the process of this invention produces a high
selectivity and high productivity of more highly unsaturated aliphatic
hydrocarbons, especially diolefins, and low selectivities and low yields
of undesirable deep oxidation products, such as carbon monoxide and carbon
dioxide. In a preferred aspect, butadiene can be produced directly from
butane in high selectivity and high productivity by the process of this
invention while maintaining low selectivities of deep oxidation products.
For the purposes of this invention, the "productivity" is defined as the
grams of unsaturated aliphatic hydrocarbon(s) produced per gram catalyst
per hour.
Unsaturated aliphatic hydrocarbons, such as monoolefins and diolefins, are
useful as monomers or comonomers in the formation of polyolefins.
Butadiene is also potentially useful as an intermediate in the preparation
of styrene.
In a second aspect, this invention is a solid heterogeneous catalyst
composition containing reactive oxygen. The composition comprises a glassy
silica matrix having a Brunauer-Emmett-Teller (BET) surface area no
greater than about 20 m.sup.2 /g and having macropores in the range from
about 500 .ANG. to about 4000 .ANG. in diameter, as determined by methods
described in detail hereinafter. The silica matrix comprises from about 25
to about 90 weight percent of the catalyst composition. Encapsulated into
the silica matrix are domains of a catalyst component comprising magnesium
oxide and molybdenum oxide. The above-identified catalyst composition
exhibits a crush strength of at least about 0.60 lb.
The catalyst composition of this invention is useful in the
above-identified process of oxidizing aliphatic hydrocarbons to more
unsaturated aliphatic hydrocarbons. Advantageously, the catalyst
composition of this invention achieves a high productivity to unsaturated
aliphatic hydrocarbons when compared with catalysts of the prior art. More
advantageously, the catalyst of this invention is strong and hard.
Consequently, the catalyst composition disclosed herein possesses the
activity and strength required for use in commercial fluid bed and
transport reactors, such as riser reactors.
In a third aspect, this invention is a process of preparing the
above-identified catalyst composition comprising (a) treating a source of
magnesium oxide with a blocking agent, (b) adding the treated source of
magnesium oxide to an alkali metal silicate solution, the silicate being
present in a concentration sufficient to provide silica in an amount
ranging from about 25 to about 90 weight percent of the catalyst
composition, (c) polymerizing the silicate to form a composite material
comprising a glassy silica matrix having a BET surface area no greater
than about 20 m.sup.2 /g and having macropores ranging from about 500
.ANG. to about 4000 .ANG. in diameter, the matrix containing domains of
the treated source of magnesium oxide, (d) ion-exchanging the composite
material with an ammonium salt to reduce the concentration of alkali metal
ions, (e) drying and calcining the composite material under conditions
sufficient to remove the blocking agent and sufficient to convert the
source of magnesium oxide into magnesium oxide, (f) impregnating the
domains of magnesium oxide with a source of an oxide of molybdenum and
optionally a promoting amount of a source of an oxide of alkali metal, (g)
calcining the resulting impregnated composite material under conditions
sufficient to convert the sources of an oxide of molybdenum and oxide of
alkali metal to an oxide of molybdenum and an oxide of alkali metal.
In a fourth aspect, this invention is a process of preparing a hard
composite material comprising a glassy silica matrix having a BET surface
area no greater than about 20 m.sup.2 /g and having macropores ranging
from about 500 .ANG. to about 4000 .ANG. in diameter, the silica matrix
having encapsulated therein domains of a metal oxide phase. The process
comprises (a) treating a source of the metal oxide with a blocking agent,
the metal oxide being selected from those which are reactive with an
alkali metal silicate, (b) adding the treated source of the metal oxide to
an alkali metal silicate solution, (c) polymerizing the silicate to form a
composite material comprising a glassy silica matrix having a BET surface
area no greater than about 20 m.sup.2 /g and having macropores ranging
from about 500 .ANG. to about 4000 .ANG. in diameter, the matrix
containing domains of the treated source of metal oxide phase, and (d)
calcining the composite material under conditions sufficient to remove the
blocking agent and sufficient to convert the source of metal oxide into
metal oxide. In this manner the above-identified hard composite material
is produced having a crush strength of at least about 0.60 lb.
The above-identified process of preparing a hard composite material is
useful for preparing a metal oxide encapsulated in silica without forming
a significant quantity of unwanted metal silicate. Thus, the process is
especially useful when the metal oxide and silica are reactive and,
without the blocking agent, would form significant quantities of metal
silicate. The composite materials are useful as strong and hard catalysts
or catalyst supports.
DETAILED DESCRIPTION OF THE INVENTION
The aliphatic hydrocarbons which can be employed in the process of this
invention include alkanes and olefins which have three or more carbon
atoms.
The alkanes can be alternatively described as paraffin hydrocarbons. These
compounds are known to those skilled in the art as saturated hydrocarbons.
As noted hereinbefore, the alkanes contain at least three carbon atoms,
and additionally, can have straight-chain or branched structures.
Typically, the alkane contains up to about 20 carbon atoms. Examples of
suitable alkanes include n-butane, n-pentane, n-hexane, n-heptane,
n-octane, n-nonane, n-decane, n-dodecane, and higher saturated homologues,
as well as isobutane, isopentane, neopentane, and likewise branched
hexanes, heptanes, octanes, nonanes, decanes, dodecanes, and higher
branched homologues. Certain alicyclic hydrocarbons are suitable
reactants, and therefore, for the purposes of this invention are included
herein. Some examples of alicyclic hydrocarbons include cyclobutane,
cyclopentane, cyclohexane, cycloheptane, cyclooctane, methylcyclopentane,
methyleyclohexane and other alkyl-substituted cycloalkanes. Preferably,
the alkane is normal or linear.
The olefins can be further described as aliphatic hydrocarbons containing
at least one unsaturated double bond. As noted earlier, the olefins should
also contain at least three carbon atoms, and typically up to about 20
carbon atoms. The location of the double bond is not critical; therefore,
the double bond can occur at a terminal or internal location along the
carbon chain. Preferably, however, the olefin has a normal or linear
structure, rather than a branched structure. For example, 1-butene is
preferred over isobutylene. Thus, some examples of suitable olefins
include, 1-butene, 2-butene, 1-pentene, 2-pentene, 3-pentene, 1-hexene,
2-hexene, 3-hexene, and likewise 1-heptene, 1-octene, 1-nonene, 1-decene,
and isomers thereof wherein the unsaturation occurs at any other position
along the carbon chain. Olefins containing more than one double bond, such
as 1,3-hexadiene and isoprene, are also acceptable, being converted in the
process of this invention to more highly unsaturated hydrocarbons. Certain
alicyclic olefins, such as cyclohexene and vinylcyclohexene, are also
suitable starting materials, and therefore, for the purposes of this
invention are included herein. Preferably, the olefin is a monoolefin.
More preferably, the olefin is 1- or 2-butene. Alkynes are not suitable
reactants for the process of this invention.
The many specific examples of aliphatic hydrocarbons, noted hereinabove,
are representative of those which are suitable for the process of this
invention, and are not intended to be limiting thereof. Other aliphatic
hydrocarbons may be available to one skilled in the art and may also be
suitable for the process of the invention.
The preferred alkanes are normal paraffins which can be represented by the
general formula:
CH.sub.3 --(CH.sub.2).sub.n --CH.sub.3
wherein n is an integer from 1 to 8. More preferably, n is an integer from
2 to 6. Most preferably, n is 2, and the alkane is n-butane.
Optionally, the aliphatic hydrocarbon reactant can be diluted with a
non-reactive gas, such as nitrogen, helium, argon, methane, carbon dioxide
or steam. While the type of diluent is determined by prevailing economic
considerations, a preferable diluent is nitrogen. If a diluent is used,
the amount can vary widely depending upon the design of the reactor and
the capacity of the solid oxidant. The hydrocarbon content of the
hydrocarbon-diluent mixture typically ranges from 1 mole percent to 100
mole percent. Preferably, the hydrocarbon content of the mixture ranges
from about 10 mole percent to about 100 mole percent, more preferably,
from about 40 mole percent to about 100 mole percent.
The catalyst composition of this invention, described in detail
hereinbelow, is a solid heterogeneous oxide at least d portion of the
oxygen of which is reactive. By this it is meant that a labile form of
oxygen is present in the catalyst, and that this labile form of oxygen is
capable of oxidizing the aliphatic hydrocarbon. Thus, in one aspect the
catalyst of this invention is a solid oxidant. After the labile oxygen is
removed through reaction, the catalyst is spent. Moreover, the catalyst
may build up over time a carbonaceous residue on its surface. The spent
and poisoned catalyst can be regenerated by contact with a source of
gaseous oxygen. Thus, in addition to the aliphatic hydrocarbon, oxygen is
required for the catalytic process of this invention.
Oxygen is typically supplied from a gaseous source provided as a continuous
oxygen-containing feed. Any source of oxygen i3 acceptable, such as pure
gaseous elemental oxygen, air, or nitrous oxide. The preferred source of
oxygen is gaseous air. Optionally, the gaseous elemental oxygen can be
diluted with a non-reactive gas, such as nitrogen, helium, argon, or
carbon dioxide. Preferably, the diluent is nitrogen. If a non-reactive
diluent is employed, the oxygen content of the mixture is preferably not
greater than about 50 mole percent. More preferably, the oxygen content of
the mixture ranges from about 0.5 mole percent to about 30 mole percent.
Most preferably, the oxygen content of the mixture ranges from about 1
mole percent to about 20 mole percent.
The amount of oxygen employed in the catalytic process of this invention is
any amount which is (1) sufficient to oxidize fully the solid
heterogeneous catalyst, and (2) sufficient to remove carbonaceous residues
from the catalyst's surface. Preferably, the regeneration of the catalyst
is carried out separately from the oxidation of the aliphatic hydrocarbon.
Alternatively, it is acceptable to co-feed a small amount of gaseous
elemental oxygen with the aliphatic hydrocarbon. The function of the
co-feed is to burn off carbonaceous residues on the surface of the
catalyst, to replenish to some extent the reactive oxygen of the catalyst,
and to burn off any hydrogen which is formed in the process. The
concentration of oxygen in the aliphatic hydrocarbon and oxygen feed is
limited by the explosive limits of this mixture. Preferably, the oxygen
concentration is maintained outside the lower explosive limit.
The solid heterogeneous catalyst composition of this invention comprises a
hard silica matrix and a catalytic component. The silica matrix can be
characterized as a glassy silica having a BET surface area no greater than
about 20 m.sup.2 /g. The term "glassy" means that the silica is an
amorphous and disordered phase, as determined by X-ray diffraction (XRD).
Additionally, the silica can be characterized as a dense phase, meaning
that it does not contain a measurable density of micropores or mesopores.
A typical micropore ranges in size from about 4 .ANG. to about 20 .ANG.,
while a typical mesopore ranges from about 20 .ANG. to about 200 .ANG..
The silica of this invention does, however, contain a random system of
macropores characterized by large pores on the order of about 500 .ANG. to
about 4000 .ANG. in diameter. In a visual sense, the topology of the
silica is best compared to that of a sponge or irregular honeycomb. The
catalytic component comprises an oxide of molybdenum and an oxide of
magnesium, at least partially combined as magnesium molybdate. Preferably,
the catalytic component consists essentially of an oxide of molybdenum and
an oxide of magnesium. The catalytic component occurs as discrete domains
of magnesium oxide containing molybdenum oxide, the domains being
encapsulated in the silica matrix. The domains of the catalyst component
range in size from about 0.1 .mu.m to about 500 .mu.m. Optionally, the
catalytic component may also contain a promoting amount of alkali metal
and/or an oxide of vanadium.
The silica in the above-identified heterogeneous catalyst acts as an inert
and hard matrix, thereby imparting a high crush strength and attrition
resistance to the catalyst so that it is suitable for use in fluid bed or
transport reactors. The magnesium oxide functions in a dual role: first,
as a support for the active catalyst component comprising magnesium oxide
and molybdenum oxide, and secondly, a3 a base. It is believed that
basicity enhances the desorption of olefinic products in the
oxydehydrogenation process. The molybdenum oxide contributes significantly
to the catalyst's activity, especially as combined with magnesium oxide in
the form of magnesium molybdate. The alkali metal promoter functions to
increase the basicity of the catalyst thereby increasing the selectivity
to higher unsaturates in the process of this invention. The alkali metal
promoter is a Group IA metal compound. Small amounts of other elements may
be present in the catalyst, provided that these elements do not materially
change the performance of the catalyst.
As a first step in preparing the catalyst composition of this invention,
magnesium oxide is encapsulated into the aforementioned silica matrix.
This preparation presents certain challenges. U.S. Pat. No. 3,678,144
teaches a method of preparing a glassy silica body having certain metal
oxides bound into the silica network. The patent is silent with respect to
magnesium oxide. It has now been discovered that when magnesium oxide
powder is blended into an aqueous potassium silicate solution with a
gellation agent according to the method of U.S. Pat. No. 3,678,144, the
aqueous silicate is readily absorbed onto the surface of the magnesium
oxide forming silica and magnesium silicates. The resulting hard composite
material exhibits significantly reduced activity in the oxydehydrogenation
process of this invention. It is believed that the reduced activity is
related to the presence of the surface silicates. Surprisingly, it has now
been further discovered that if good phase separation exists between the
magnesium oxide and silica, it is possible to maintain an active magnesium
oxide surface.
In view of the above and in another aspect, this invention is a method of
preparing a composite material comprising a glassy silica matrix having
encapsulated therein domains of magnesium oxide. The aforementioned method
is easily generalized for preparing a glassy silica matrix having
encapsulated therein discrete domains of a reactive metal oxide phase. The
term "reactive" means that the metal oxide or a source of the metal oxide
is capable of reacting with the alkali metal silicate from which the
silica is derived or reacting with silica itself to form metal silicates.
The method of this invention comprises (a) treating a source of a metal
oxide with a blocking agent, the metal oxide being selected from those
which are reactive with an alkali metal silicate, (b) adding the treated
source of metal oxide to an alkali metal silicate solution, (c)
polymerizing the silicate to form a composite material comprising a glassy
silica matrix having a BET surface area no greater than about 20 m.sup.2
/g and having macropores ranging in size from about 500 .ANG. to about
4000 .ANG., the silica matrix having encapsulated therein domains of the
source of metal oxide treated with blocking agent, and (d) calcining the
composite material under conditions sufficient to remove the blocking
agent and sufficient to convert the source of metal oxide into metal
oxide. Optionally, the composite material may be ion-exchanged with an
ammonium salt after the polymerization step (Step c) and prior to the
calcination step (Step d) to reduce the concentration of alkali metal
ions. Advantageously, in this preparative process the formation of
deactivating surface silicates is significantly reduced. Moreover, good
phase separation exists between the metal oxide and silica when compared
with the process of U.S. Pat. No. 3,678,144 which does not employ blocking
agent.
Any source of metal oxide is suitable for the preparation of the composite
material provided that the metal oxide itself is reactive with an alkali
metal silicate. The metals of Groups IIA, IIIA, IVA, and VA provide
suitable reactive oxides, the group designations (IIA, IIIA, etc.)
following the recommendations of the former IUPAC. Preferably, the metals
are selected from the group consisting of magnesium, titanium, zirconium
and niobium. More preferably, the metal is magnesium. Aside from the
oxides themselves, suitable sources of such oxides include the hydroxides,
halides, nitrates, sulfates, acetates, and carbonates of the selected
metal. Preferred sources include the metal oxides and hydroxides. Even
more preferably, the source of metal oxide is an oxide or hydroxide of
magnesium, titanium, niobium or zirconium. Most preferably, the source of
metal oxide is magnesium hydroxide or magnesium oxide. It is also
beneficial for the particle size of the magnesium hydroxide to range from
about 0.1 .mu.m to about 500 .mu.m, preferably, from about 1 .mu.m to
about 250 .mu.m.
The blocking agent may be any organic compound with a plurality of
functional groups containing oxygen or nitrogen. Non-limiting examples
include polyols, poly(carboxylic acids), polyanhydrides, polyamines,
polyamides, polyesters, polyethers, and other polyhydroxylated compounds,
such as cellulosies and starches. Polymers based on phenolic or
phenolformaldehyde resins may also be used. Preferred blocking agents
include poly(vinyl alcohol) and polyacrylic and polymethacrylic acids or
salts. More preferred is poly(vinyl alcohol) having a molecular weight
ranging from about 1000 to about 500,000. Most preferred is poly(vinyl
alcohol) having a molecular weight ranging from about 14,000 to about
115,000, available as 75-100 percent hydrolyzed acetate groups.
Typically, the blocking agent is dissolved in a suitable solvent to form a
solution, and the source of metal oxide is mixed into the solution to form
a second solution or gel or paste. Any solvent is acceptable provided that
it is inert with respect to the blocking agent and source of metal oxide.
Water is the preferred solvent, but acetone, alcohols, and other common
organic solvents are also acceptable. The concentration of the blocking
agent in the solvent usually ranges from about 1 weight percent to about
50 weight percent. The source of metal oxide is generally added slowly and
with a high degree of agitation to the solution containing the blocking
agent. The amount of blocking agent employed typically ranges from about 1
to about 20 weight percent of the weight of the source of metal oxide. The
resulting solution or gel or paste is dried at a temperature in the range
from about 50.degree. C. to about 200.degree. C. for a time sufficient to
remove the solvent and form a dried solid. Thereafter, the solid is
crushed and sieved to a fine powder. At this stage, a transmission
electron micrograph (TEM) of the powder typically reveals that some of the
particles of the source of metal oxide are coated with a layer of blocking
agent, the thickness commonly ranging from about 0.1 .mu.m to about 1
.mu.m. Other particles, however, do not show any coating, and it is
believed that the coating is thinner than the detectable limit, possibly
on the order of one monolayer in thickness.
After the source of metal oxide is treated with blocking agent, the treated
source is blended into an aqueous alkali metal silicate solution and the
silicate is polymerized. Suitable alkali metal silicate solutions and
polymerization conditions are specified in U.S. Pat. No. 3,678,144, and
therefore the relevant sections of that patent are incorporated herein by
reference. For example, the suitable alkali silicates include lithium
silicate, sodium silicate, and potassium silicate. In order to maintain
the silica in solution, the concentration of the alkali metal must be
sufficient to yield a solution having a pH greater than about 10.
Preferably, the alkali silicate solution is a potassium silicate solution,
more preferably, a commercially available potassium silicate solution
containing 8.3 weight percent K.sub.2 O and 20.0 weight percent SiO.sub.2,
the balance being water. Optionally, colloidal silica may be used in
combination with the alkali silicate solution. The amount of colloidal
silica which may be blended with the alkali silicate ranges form about 0
to about 30 weight percent of the total silica present.
The metal oxide source, treated with blocking agent, is blended into the
alkali silicate solution very slowly and with a high degree of agitation
to ensure that the solution remains smooth and fluid. The amount of alkali
silicate solution, and optional colloidal silica, employed is sufficient
to provide silica in the range from about 25 to about 90 weight percent
based on the weight of the calcined composite material, preferably from
about 35 to about 70 weight percent. The actual value will vary depending
upon the end use of the Composite material. In the preferred application
involving a catalyst containing magnesium and molybdenum oxides for butane
oxidation, the silica concentration ranges from about 25 to about 90
weight percent based on the weight of the calcined catalyst composition.
A gellation agent is required for the polymerization of the silicate. The
gellation agent functions to reduce the pH of the silicate solution by
neutralizing the alkali metal ions which are present, and thereafter the
silica polymerizes. Suitable gellation agents include formamide,
formaldehyde, paraformaldehyde, glyoxal, ethyl acetate, and methyl
acetate. Preferably, the gellation agent is formamide. Since the rate of
polymerization varies with the specific gellation agent, it may be added
to the alkali silicate solution either before or after the addition of the
treated metal oxide source. If the gellation agent is added first, then
the polymerization should not reach completion before the metal oxide
source is fully blended. For example, if the gellation agent is formamide,
it is usually added to the silicate solution prior to the addition of
metal oxide. If the gellation agent is ethyl acetate, it should be added
after the addition of metal oxide. The concentration of gellation agent is
related to the concentration of alkali ions present. Typically, the
concentration ranges from about 1 to about 10 weight percent based on the
weight of the alkali silicate solution, preferably from about 2 to about 5
weight percent.
There are different ways of handling the viscous mixture containing the
alkali silicate, the treated metal oxide source and the gellation agent.
In one method, the mixture is heated in a batch in a drying oven typically
under a nitrogen purge at a temperature ranging from about 70.degree. C.
to about 120.degree. C. Normally the mixture sets to a hard mass within at
least about 1 hour, at which time it may be broken into smaller pieces and
cured. The curing process generally includes heating at a temperature in
the range from about 100.degree. C. to about 225.degree. C. for a time
ranging from about 2 hr to about 10 hr. Post cure, the dried composite is
usually crushed and sieved to a powder having a particle size in the range
from about 177 .mu.m to 1190 .mu.m (80 to 14 mesh). The particles of dried
powder comprising the treated source of metal oxide encapsulated in the
above-identified matrix of silica, are typically irregular in shape.
Alternatively, the viscous mixture containing the treated source of metal
oxide, the gellation agent, and the alkali silicate may be suspension
polymerized to yield spheroidal beads or balls having a size in the range
from about 200 .mu.m to about 1500 .mu.m. Spheroidal particles are
preferred for fluid-bed transport reactors. In this method, the mixture is
added to an immiscible liquid, typically a chlorinated hydrocarbon, such
as The Dow Chemical Company's DOWTHERM E.RTM. o-dichlorobenzene, at a
temperature in the range from about 5.degree. C. to about 100.degree. C.,
preferably from about 10.degree. C. to about 80.degree. C. The addition
may be effected by simply pouring the mixture into the immiscible liquid
with sufficient agitation to disperse the mixture into droplets or by
injecting the mixture through a droplet-forming nozzle. In order to
prevent coalescence of the spheres, fumed silica may be added as a
suspension agent to the chlorinated hydrocarbon. Bead size is controlled
by the stirring rate of the shear mixer. Typically, a shear rate of about
300 rpm to about 725 rpm is used. This method yields hard, spheroidal
beads comprising regions of the treated source of metal oxide isolated
within the above-described silica matrix.
As a third alternative, the viscous mixture containing the treated source
of metal oxide, the gellation agent, and the alkali silicate can be
spray-dried to form spheroidal particles ranging in diameter from about 10
.mu.m to about 250 .mu.m. For industrial scale applications the
spray-drying method is preferred. Any spray-drying equipment which is
conventionally used to produce catalyst particles for fluidized bed
reactors may be employed. For example, a Niro Atomizer S-12.5R/N spray
drying apparatus, with a means for controlling the inlet and outlet
temperatures, is acceptable.
Analysis of the composite material following polymerization of the silicate
reveals good phase separation between the source of metal oxide and the
silica matrix. For example, a backscattered electron image of a material
produced by the polymerization of silicate in the presence of poly(vinyl
alcohol)-blocked magnesium hydroxide reveals a silica/magnesium hydroxide
composite. The corresponding elemental Mg map shows areas of high
magnesium concentration which are identified as discrete magnesium
hydroxide particles. The corresponding elemental Si map reveals that
essentially no silicon resides in areas of high magnesium concentration.
Additionally, potassium levels are much higher in the silicon rich areas
than in areas of high magnesium concentration, as illustrated by elemental
K mapping. From these data it is concluded that good separation of the
magnesium hydroxide and silica phases is Present. Transmission electron
micrographs of the above-identified magnesium hydroxide/silica composite
show predominantly crystalline magnesium hydroxide bounded by a dense,
glassy silica. Again, good phase separation exists for at least about 80
percent of the composite. Up to 20 percent of the silica may appear as
crystalline fines, which may contain some magnesium; however, not enough
magnesium is present to indicate formation of magnesium silicate.
If desired, the composite can be leached or treated with solvents to remove
the metal oxide from the silica matrix to yield a pure silica matrix. This
procedure simply requires that the composite be soaked in an acid
solution. In the absence of the domains of metal oxide, the silica gives
the appearance of a sponge or irregular honeycomb. The BET surface area of
the silica is no greater than about 20 m.sup.2 /g, preferably no greater
than about 10 m.sup.2 /g, more preferably no greater than about 5 m.sup.2
/g. At the lower limit it is possible for the surface area to be as low as
0.2 m.sup.2 /g. The BET method for determining surface area is described
by R. B. Anderson in Experimental Methods in Catalytic Research, pp.
48-66, Academic Press, 1968. As noted hereinbefore, the silica matrix
essentially does not contain a microporous or mesoporous structure;
however, a large macroporous structure randomly permeates the matrix. The
macropores range in diameter from 500 .ANG. to about 4000 .ANG., as
determined by mercury infusion techniques using, for example, a
Micromeritics Model 9305 mercury porosimeter.
The composite comprising the silica matrix and the treated metal oxide may
contain alkali metal ions derived from the alkali silicate solution.
Accordingly, the composite will have basic properties. Should a less
basic, neutral or acidic composite be desired, the composite may be
ion-exchanged with an acid solution or an ammonium salt, such as ammonium
nitrate, to the desired degree of acidity. In the case of the catalyst
composition of this invention, the concentration of alkali metal ions may
be reduced via ion-exchange to levels less than about 0.5 weight percent,
preferably, less than about 0.1 weight percent. The ion-exchange procedure
is conducted after polymerization of the silicate (Step c) and prior to
calcination (Step d). The molarity of the acid or ammonium salt solution
is typically low, preferably ranging from about 0.1 M to about 2 M. The pH
of the solution is typically in the range from about 7.5 to about 9.0,
preferably in the range from about 8.2 to about 8.9. The ion-exchange
procedure may be carried out simply by stirring the composite in a flask
filled with the ion-exchange solution or by passing the solution through a
column filled with composite. At least two ion-exchanges are preferred,
and more may be beneficial.
Following the optional removal of alkali ions, the composite is dried for
about 2 hr to about 10 hr at a temperature between about 60.degree. C. and
about 150.degree. C. Thereafter, the composite is calcined at a
temperature ranging from about 400.degree. C. to about 800.degree. C. for
a period of about 1 hr to about 10 hr to remove the blocking agent and to
convert the source of metal oxide to the metal oxide. After calcination a
composite material is obtained comprising the above-described silica
matrix having encapsulated therein discrete regions of metal oxide phase.
Calcination does not significantly change the morphology or surface area
of the silica matrix. For the specific case of magnesium oxide, the BET
surface area of the magnesium oxide phase ranges from about 70 m.sup.2 /g
to about 170 m.sup.2 /g. Accordingly, the calcined composite material has
a BET surface area ranging from about 30 m.sup.2 /g to about 150 m.sup.2
/g.
The calcined composite comprising the silica matrix and metal oxide can be
impregnated with any catalytic metal or metal compound to form a hard
catalyst composition. For example, a composite comprising the silica
matrix and magnesium oxide can be impregnated with a solution containing a
source of molybdenum oxide to form a strong catalyst composition which is
active in the hydrocarbon oxydehydrogenation process Of this invention.
The impregnation technique is described by Charles N. Satterfield in
Heterogeneous Catalys is in Practice, McGraw-Hill Book Company, New York,
1980, pp. 82-83, which is incorporated herein by reference. Any source of
molybdenum oxide is acceptable, including for example, MoO.sub.3,
(NH.sub.4).sub.2 Mo.sub.2 O.sub.7, (NH.sub.4).sub.6 Mo.sub.7 O.sub.24
.multidot.4H.sub.2 O, and (NH.sub.4).sub.2 MoO.sub.4. The molybdenum oxide
can also be obtained from a precursor molybdenum compound, such as
molybdenum carbonyls, e.g., MoO(CO).sub.6. Preferably, the molybdenum is
in the +6 oxidation state. Preferably, the source of molybdenum oxide is
ammonium heptamolybdate represented by the formula (NH.sub.4).sub.6
Mo.sub.7 O.sub.24 .multidot. 4H.sub.2 O. Generally, the desired quantity
of a molybdenum oxide or precursor compound is dissolved in a solvent,
preferably water, to make a solution. The solution is brought into contact
with the composite material and the resulting slurry is dried to remove
solvent. If the solution is aqueous, the drying is conducted in an oven at
a temperature in the range from about 70.degree. C. to about 120.degree.
C. Thereafter, the dried slurry is calcined to form a catalytically active
composition containing the silica matrix, magnesium oxide and molybdenum
oxide. The calcination is typically conducted at a temperature ranging
from about 300.degree. C. to about 900.degree. C. for a time ranging from
0.5 hour to about 24 hours. Preferably, the calcination is conducted at a
temperature in the range from about 500.degree. C. to about 800.degree.
C., more preferably, from about 550.degree. C. to about 650.degree. C.
Alternatively, the dried slurry, described hereinabove, can be employed
directly with no prior calcination in the catalytic process of this
invention. Since the molybdenum precursor can be converted into molybdenum
oxide at or about 300.degree. C., and since the catalyst bed is heated to
a temperature higher than about 300.degree. C., the dried composition will
be converted in situ into the catalytically active magnesium and
molybdenum oxides. As noted hereinbefore, calcination essentially does not
change the basic morphology of the composite. The molybdenum oxide is
associated with the magnesium oxide particles and not with the silica
matrix, as shown by TEM.
The elemental analysis of the calcined solid reveals a composition ranging
from about 3 weight percent MoO.sub.3 to about 30 weight percent
MoO.sub.3, from about 72 weight percent MgO to about 7 weight percent MgO,
and from about 25 weight percent silica to about 90 weight percent silica.
Preferably, the composition ranges from about 5 weight percent MoO.sub.3
to about 25 weight percent MoO.sub.3, from about 25 weight percent MgO to
about 70 weight percent MgO, and from about 25 weight percent silica to
about 70 weight percent silica. More preferably, the composition ranges
from about 10 weight percent MoO.sub.3 to about 20 weight percent
MoO.sub.3, from about 30 weight percent MgO to about 55 weight percent
MgO, and from about 35 weight percent silica to about 50 weight percent
silica.
It is beneficial to add a promoting amount of at least one alkali metal
promoter to the catalyst component. The promoter serves to increase the
selectivity and productivity of unsaturated products, e.g. diolefins, in
the process of this invention. Such a promoter is typically a compound of
lithium, sodium, potassium, rubidium, cesium or francium of sufficient
basicity to improve the selectivity to higher unsaturates in the process
of this invention. Suitable compounds include the alkali oxides,
hydroxides and carbonates. Compounds which decompose on heating to the
oxides are also suitable, such as alkali metal acetates and oxalates.
Alkali metal salts may be found which are also suitable, although
typically, the alkali metal halides and alkali metal silicates are not
preferred due to their lower basicity. Preferably, the alkali metal
promoter is an alkali metal oxide, hydroxide, carbonate, acetate, or
oxalate. More preferably, the alkali metal promoter is an oxide or
hydroxide of potassium or cesium. Most preferably, the alkali metal
promoter is an oxide or hydroxide of potassium.
The amount of alkali metal promoter significantly affects the selectivity
of the catalyst. Generally, any amount of alkali metal promoter is
acceptable which is sufficient to increase the selectivity and the
productivity of unsaturated products, such as diolefins, in the process of
this invention. Typically, the amount of alkali metal promoter calculated
as the alkali hydroxide is in the range from about 0.01 weight percent to
about 5 weight percent based on the combined weights of silica, magnesium
oxide and molybdenum oxide. Preferably, the amount of alkali metal
promoter calculated as the alkali metal hydroxide is in the range from
about 0.02 weight percent to about 2 weight percent, more preferably, in
the range from about 0.1 weight percent to about 1.0 weight percent, based
on the combined weights of silica, magnesium oxide and molybdenum oxide.
Below the lower preferred amount of alkali metal promoter the selectivity
to diolefin is reduced while the selectivity to deep oxidation products is
increased. Above the upper preferred amount of alkali metal promoter the
selectivity and productivity to diolefin are also reduced.
The alkali metal promoter can be added to the catalyst component in a
variety of ways known to those in the art. For example, the promoter can
be applied by the impregnation technique, noted hereinbefore. In this
technique the molybdenum-impregnated composite is immersed in a solution
of the alkali metal promoter, for example, a methanolic solution of the
alkali metal oxide or hydroxide. The alkali-impregnated composite is then
drained of excess solution, dried in an oven to remove residual solvent,
and calcined at a temperature in the range from about 550.degree. C. to
about 650.degree. C. Alternatively, the alkali metal compound can be
impregnated from the same solution as the molybdenum compound.
Optionally, the catalyst component of this invention can contain an
activator which functions to increase the activity of the catalyst at any
given temperature. Preferably, the activator does not decrease
significantly the selectivity to diolefins and monoolefins. Preferably,
the activator allows the reaction to be run at a lower temperature, while
achieving high selectivity and high productivity of diolefins. Activators
which are suitable for incorporation into the catalyst include the oxides
of vanadium, preferably V.sub.2 O.sub.5. Any amount of vanadium oxide can
be added to the catalyst provided that (1) the activity of the catalyst is
increased, and (2) the selectivity for alkenes, including mono- and
diolefins, is not significantly decreased. Generally, if an activator is
used, the concentration ranges from about 0.05 weight percent to about 10
weight percent based on the total weight of the catalyst composition.
Preferably, the concentration of activator ranges from about 0.10 weight
percent to about 5.0 weight percent, more preferably, from about 0.15
weight percent to about 2.0 weight percent. The activator can also be
applied to the composite by the impregnation technique.
The process of this invention can be carried out in any suitable reactor,
including batch reactors, continuous fixed-bed reactors, surry reactors,
fluidized bed reactors, and riser reactors. Preferably, the reactor is a
continuous flow reactor, such as a continuous fixed-bed reactor or a
transport reactor of the type described hereinafter.
The preferred commercial reactor for the process of this invention is a
transport bed reactor, such as a riser reactor. In such reactors the
catalyst particles are subjected to constant impact with other catalyst
particles and with the walls of the reactor. Such forces gradually reduce
the size of the catalyst particles to small fines which are lost in the
reaction products; thus, the useful lifetime of the catalyst is greatly
limited. Consequently, it is required for the catalyst to be prepared in a
form which is able to withstand high impact and erosion forces. The
catalyst composition of this invention possesses the strength and
attrition resistance required for commercial use.
Typically, the riser reactor comprises an upright vessel of relatively low
ratio of diameter to length. The catalyst is continuously charged into the
bottom of the riser reactor. Likewise, the aliphatic hydrocarbon
feedstream is delivered concurrently to the bottom of the riser reactor as
a vapor phase feed or as a liquid phase feed. Preferably, the alkane is
delivered as a vapor phase feed premixed with an inert, gaseous diluent,
and optionally, a small concentration of oxygen. The feed moves upward
through the reactor, thereby contacting the catalyst. Upon contacting the
catalyst, the feed is converted into a mixture of products, including
monoolefins, diolefins, higher unsaturated olefins, cracking products,
deep oxidation products, such as carbon monoxide and carbon dioxide, and
heavies, such as benzene and furan in the case of a butane feed. The
product stream exits the riser reactor and is separated by known methods,
such as distillation, to recover the desired products, typically the
diolefins. Unreacted alkanes and monoolefin products are recycled to the
riser reactor for further oxidation.
Riser reactor technology is advantageous for the process of this invention,
because (1) the hazard of using a feedstream containing a mixture of
alkane and/or olefin and elemental oxygen is eliminated, and (2) the
selectivity for diolefins is enhanced, especially at the high temperatures
required for this process. In contrast, if a feedstream of alkane and
oxygen is employed at a high temperature and a high oxygen/alkane mole
ratio, there is a tendency to produce more deep oxidation products, such
as carbon monoxide and carbon dioxide. In addition, the danger of a
run-away reaction is greater.
The operation of a riser reactor can be simulated by employing a method of
alternating pulses. Thus, a pulse of the hydrocarbon-containing feed is
passed through the catalyst bed where it is oxidized to form the desired
olefin products. Next, a pulse of inert gas is passed through the catalyst
bed to purge the bed of residual alkanes and alkenes. After purging, a
pulse of oxygen-containing feed is passed through the catalyst bed to
regenerate the catalyst. Finally, a second pulse of inert gas is passed
through the catalyst bed to purge the bed of oxygen, after which the cycle
is repeated. Such a procedure is employed in the illustrative embodiments,
described hereinafter.
The aliphatic hydrocarbon reactant is contacted with the catalyst at any
operable temperature which promotes the oxidation process of this
invention and yields the desired unsaturated products. Typically, the
temperature is in the range from about 400.degree. C. to about 700.degree.
C. Preferably, the temperature is in the range from about 500.degree. C.
to about 650.degree. C. More preferably, the temperature is in the range
from about 530.degree. C. to about 600.degree. C. Below the preferred
lower temperature the conversion of reactant may be low. Above the
preferred upper temperature the selectivity and productivity of diolefin
products may decrease.
Likewise, the aliphatic hydrocarbon reactant is contacted with the catalyst
at any operable pressure which promotes the oxidation process of this
invention and yields the desired unsaturated products. Typically, the
partial pressure of the reactant is adjusted to maintain the reactant in
the vapor state at the operating temperature. Preferably, the partial
pressure of the aliphatic hydrocarbon is in the range from about
subatmospheric to about 100 psig. More preferably, the partial pressure is
in the range from about 1 psig to about 30 psig. Most preferably, the
partial pressure is in the range from about 3 psig to about 15 psig.
When the process of this invention is conducted in a continuous flow
reactor, described hereinbefore, the flow rate of the reactants can be
varied. Generally, in the process of this invention the aliphatic
hydrocarbon reactant is fed into the reactor at any operable flow rate
which promotes the oxidation reaction and yields the desired conversion
and selectivity of unsaturated products. The flow rate is expressed as the
gas hourly space velocity (GHSV) and is given in units of volume of
aliphatic hydrocarbon-containing gaseous feed per total reactor volume per
hour or simply hr.sup.-1. Typical values vary from about 100 hr.sup.-1 to
about 20,000 hr.sup.-1. Preferably, the GHSV ranges from about 100
hr.sup.-1 to about 500 hr.sup.-1. It should be understood that the space
velocity controls the residence time of the reactants. In a riser reactor,
for example, a gas residence time less than about 10 seconds is preferred,
while times less than about 5 seconds are more preferred and less than
about 1 second are most preferred.
For the case of the riser reactor, the spent catalyst leaves the top of the
reactor and is transported into a second reactor for regeneration.
Regeneration is effected by contact with oxygen. Typically, a preheated
oxygen source, like that described hereinbefore, is fed into the bottom of
the second reactor. The spent catalyst is contacted with the oxygen source
at any operable temperature, pressure, and oxygen-source flow rate which
are sufficient to regenerate the catalyst. The process variables should be
controlled, however, so as to prevent a runaway reaction or the buildup of
excessive heat. Preferably, the temperature is in the range from about
500.degree. C. to about 700.degree. C., more preferably, in the range from
about 550.degree. C. to about 650.degree. C. Preferably, the pressure is
in the range from subatmospheric to about 100 psig, more preferably, in
the range from about 2 psig to about 50 psig. The oxygen-source flow rate
will depend upon the heat transfer properties of the particular reactor.
For example, at some high flow rates the temperature may rise dramatically
resulting in an uncontrolled reaction.
When the aliphatic hydrocarbon is contacted with the catalyst of this
invention, an oxidation of the aliphatic hydrocarbon occurs resulting in
the loss of at least two hydrogen atoms from the hydrocarbon reactant with
formation of by-product water. The organic products which are produced are
predominantly unsaturated aliphatic hydrocarbons, such as monoolefins and
diolefins. These unsaturated products usually contain the same number of
carbon atoms as the reactant aliphatic hydrocarbon. Thus, these products
are not products of cracking, which would contain fewer carbon atoms than
the starting hydrocarbon. Generally, also, the unsaturated products
possess a higher degree of unsaturation than the reactant hydrocarbon. For
example, alkane3, such as butane, can lose two hydrogen atoms to yield
monoolefins, such as 1-butene, trans-2-butene, and cis-2-butene. In turn,
monoolefins, such as the butenes previously cited, can lose two hydrogen
atoms to form 1,3-butadiene.
The preferred diolefin products can be represented by the general formula:
CH.sub.2 .dbd.CH--CH.dbd.CH--(CH.sub.2).sub.m --H
wherein m is an integer from 0 to about 6. Preferably, m is an integer from
0 to about 2. More preferably, m is 0 and the unsaturated product is
1,3-butadiene. Isomers of the formula shown hereinabove can also be formed
wherein the unsaturation occurs at any other location along the carbon
chain. Preferably, the unsaturation occurs in a conjugated fashion, as
exemplified in the product 1,3-butadiene. Even more unsaturated variants
of the general formula can be formed wherein further oxidation has
occurred to yield more than two ethylenic double bonds. Alkynes, however,
are not formed in significant amounts.
In addition to alkenes, the product stream can contain by-products of
various types. For example, when the saturated alkane is n-butane, small
quantities of cracking products, such as propylene and ethylene, can be
formed, as well as heavies, such as benzene and furan, and deep oxidation
products, such as carbon monoxide and carbon dioxide. Unexpectedly,
however, these by-products, especially the deep oxidation products, are
significantly reduced over the prior art processes.
For the purposes of this invention, "Conversion" is defined as the mole
percentage of aliphatic hydrocarbon reactant lost from the feed stream as
a result of reaction. The conversion can vary widely depending upon the
reactants, the form of the catalyst, and the process conditions such as
temperature, pressure, flow rate, and catalyst residence time. Within the
preferred temperature range, as the temperature increases the conversion
generally increases. Within the preferred gas hourly space velocity range,
as the space velocity increases the conversion generally decreases.
Typically, the conversion of the aliphatic hydrocarbon is at least about
10 mole percent. Preferably, the conversion is at least about 20 mole
percent; more preferably, at least about 30 mole percent; even more
preferably, at least about 40 mole percent; and most preferably, at least
about 50 mole percent.
Likewise, for the purposes of this invention "selectivity" is defined as
the mole percentage of converted carbon which forms a particular product.
Selectivities also vary widely depending upon the reactants, the form of
the catalyst, and the process conditions. Typically, the process of this
invention achieves high selectivities to diolefins. Within the preferred
temperature range, as the temperature increases the selectivity for
alkenes generally decreases. Within the preferred space velocity range, as
the space velocity increases the selectivity for alkenes generally
increases. Preferably, the combined selectivity to all alkenes is at least
about 50 mole percent; more preferably, at least about 60 mole percent;
even more preferably, at least about 70 mole percent; most preferably, at
least about 80 mole percent. Typically, the selectivity to diolefins is at
least about 40 mole percent. Preferably, the selectivity to diolefins is
at least about 50 mole percent, more preferably, at least about 60 mole
percent, most preferably, at least about 70 mole percent.
The concept of simultaneous high conversion and high selectivity can be
conveniently expressed in terms of yield. For the purposes of this
invention, the term "yield" refers to the numerical product of the
single-pass conversion and selectivity. For example, a process according
to the present invention operating at a conversion of 0.65, or 65 mole
percent, and a selectivity to diolefin of 0.75, or 75 mole percent, would
have a diolefin yield of 0.49, or 49 mole percent. Typically, the yield of
diolefin achieved in the process of this invention is at least about 8
mole percent. Preferably, the yield of diolefin achieved in the process of
this invention is at least about 18 mole percent, more preferably at least
about 28 mole percent, most preferably, at least about 35 mole percent.
Typically, in the oxidation of butane the yield of total C.sub.4 olefins
is at least about 20 mole percent. Preferably, in the oxidation of butane
the yield of total C.sub.4 olefins is at least about 30 mole percent, more
preferably, at least about 35 mole percent, most preferably, at least
about 40 mole percent.
The rate at which a desired product is produced in the process of this
invention can be expressed in the concept of space-time yield. For the
purposes of this invention the "space-time yield" is defined as the mole
percentage yield of a given product per hour (yield hr.sup.-1), and it is
the numerical product of the single-pass conversion, the selectivity, the
gas hourly space velocity, and the concentration of the aliphatic
hydrocarbon in the feedstream, wherein the conversion, selectivity and
concentration are expressed as decimal fractions. Preferably, the
space-time yield of diolefin in the process of this invention for a 20
volume percent alkane feed is at least about 30 percent per hour, more
preferably, at least about 60 percent per hour, and most preferably, at
least about 80 percent per hour.
Another measure of the rate at which a desired product is produced is the
"productivity," defined as the grams unsaturated aliphatic hydrocarbon(s)
formed per gram catalyst per hour (g/g cat-hr). Preferably, the
productivity of butadiene in this process is at least about 0.10 g/g
cat-hr, more preferably, at least about 0.25 g/g cat-hr. Preferably, the
combined productivities of all of the unsaturated aliphatic hydrocarbons,
such as C4 olefins, is at least about 0.15 g/g cat-hr, more preferably, at
least about 0.20 g/g cat-hr, most preferably, at least about 0.30 g/g
cat-hr.
Illustrative Embodiments
Testing the attrition resistance of a catalyst requires having on hand a
large amount of catalyst sample. It is desirable to have a simple test
procedure for small catalyst samples which gives an indication of
attrition resistance. A test of crush strength is such a procedure,
because increased crush strength suggests better attrition resistance.
Crush strength can be tested on any conventional equipment designed for
such a purpose, however, a materials testing frame capable of providing a
constant crosshead movement rate and a load capacity of at least 50 lb is
preferred. For example, a suitable testing frame is an Instron 1125
instrument with a 20,000 lb capacity. This frame can be equipped with a
200 lb compression load cell with a stainless steel compression platen. A
1 cm diameter compression jig is designed and built to screw in directly
to the bottom portion of the machine crosshead. A strip chart or computer
data acquigition system is suitable for monitoring the load versus
crosshead displacement.
Prior to testing, the load cell is balanced and calibrated. This is
completed with the cell/platen in the compression testing configuration.
The load cell is allowed to equilibrate for at least 15 minutes prior to
calibration. Preferred instrument settings are the following: crosshead
speed, 0.02 inches/min; chart speed, 2.0 inches/min; load cell range
setting, 0-10 1b full scale. The specimen is centered on the load cell
platen just below the compression jig. The crosshead is carefully lowered
by manual control until minimal clearance between the fixture and specimen
is achieved. Each specimen is tested at room temperature until the first
sign of failure is observed (drop in load). The maximum load observed by
the specimen is determined by the strip chart or computer data system.
The composite material or catalyst composition to be tested is sized into
particles ranging from about 500 .mu.m to about 800 .mu.m. These particles
are calcined at 600.degree. C. for 2 hours prior to testing. Care should
be taken to select particles of similar size for testing, and regular
shaped particles are preferred. Typically, a minimum of ten specimens is
tested for each sample. The crush strength of the catalyst of this
invention is typically at least about 0.60 lb, preferably, at least about
0.80 lb, more preferably, at least about 1.00 lb, and most preferably, at
least about 1.25 lb, as measured on a particle having a size in the range
from about 500 .mu.m to about 800 .mu.m.
The following examples are illustrative of the process and catalyst of this
invention, but are not intended to be limiting thereof. All percentages
are given in mole percent carbon, unless noted otherwise.
EXAMPLE 1 --COMPOSITE MATERIAL AND CATALYST PREPARATION
A. Preparation of the Composite Material
A 5 weight percent poly(vinyl alcohol) (PVA) solution is prepared by adding
PVA (26 g; MW 115,000; 100 percent hydrolyzed ester) to cold water (500 g)
with rapid stirring and heating to 90.degree. C. Magnesium hydroxide
powder (90 g) is added to the PVA solution (200 g) with rapid mechanical
stirring to form a creamy suspension. The suspension is dried in a
nitrogen-purged oven at 80.degree. C. for 18 hr, and the resulting
PVA-treated magnesium hydroxide solid is rough crushed and heated further
at 125.degree. C. for 4 hr. The dried solid is fine crushed to pass a 170
mesh screen (88 .mu.m).
With rapid mechanical stirring, formamide (3 g) is added slowly to a
potassium silicate solution (100 g; 20.8 weight percent SiO.sub.2, 8.3
weight percent K20) to form a clear solution free of gel clusters. The
PVA-treated magnesium hydroxide powder (50 g), prepared hereinabove, is
added gradually to the silicate solution to form a well-mixed slurry. The
slurry is poured into a plastic beaker, covered with a watch glass to slow
evaporation, and placed in an oven at 80.degree. C. for about 45 minutes.
During this time, the silicate polymerizes in the batch taking the form of
the beaker. The polymerized material is removed and cut into chunks which
are cured and dried for 18 hr at 80.degree. C. The hardened chunks are
crushed to a size ranging from about 177 .mu.m to about 1190 .mu.m (80-14
mesh). The crushed particles (70 ml) are loaded into a column and washed
four times with 150 ml portions of an aqueous ammonium nitrate solution (1
M; pH 8). The wet particles are then slurried twice in 1 M ammonium
nitrate, filtered, slurried twice with acetone, and filtered again. This
procedure is designed to remove water located in the pores which could
fracture the particles during heating. The filtered particles are air
dried at room temperature and dried further at 80.degree. C. for 6 hr.
Elemental analysis of the particles indicates that the potassium level is
less than 0.1 weight percent. The particles are further dried and calcined
as follows: 2 hr at 100.degree.-150.degree. C., 4 hr at
150.degree.-300.degree. C., 1 hr at 300.degree.-400.degree. C., 4 hr at
400.degree.-450.degree. C., 2 hr at 450.degree.-600.degree. C., and 4 hr
at 600.degree.-610.degree. C. A composite material is obtained comprising
a silica matrix having domains therein of magnesium oxide, as determined
by TEM. The silica matrix is characterized as having a BET surface area of
1 m.sup.2 /g and a random macropore system wherein the diameter of the
pores is in the range from about 3000 .ANG. to about 4000 .ANG.. The
domains of magnesium X-1 oxide exhibit a BET surface area of 140 m.sup.2
/g.
B. Preparation of the Catalyst
An aqueous solution containing 25 weight percent ammonium heptamolybdate
(AHM) (23 g, 20 weight percent as MoO.sub.3) adjusted to pH 8.5 is added
to the composite material prepared hereinabove (30 g). The wetted material
is dried overnight in flowing nitrogen at 80.degree. C. and then calcined
in air as follows: 2 hr at 100.degree.-150.degree. C., 4 hr at
150.degree.-600.degree. C., and 4 hr at 600-6100C to yield a catalyst
composition comprising the above-identified silica matrix having domains
therein of magnesium oxide containing molybdenum oxide. The catalyst
contains 40.00 weight percent SiO.sub.2, 16.67 weight percent MoO.sub.3,
the remainder being MgO. The crush strength of the catalyst, as measured
on an Instron #IV crush strength instrument, gives a maximum load of 1.38
.+-.0.44 lb for spheroidal particles of 600 Jim size. By comparison,
commercial alumina beads of approximately the same size, which are
suitable for use in a transport reactor, exhibit a maximum load of
1.53.+-.0.64 lbs. Thus, the strength of the catalyst composition of this
invention is sufficient for use in a transport reactor.
EXAMPLE 2 --BUTANE OXIDATION
A catalyst similar to the one prepared in Example 1(B) is employed in the
oxidation of butane in the following manner: approximately 15 cc of
catalyst are loaded into a Vycoro reactor tube (18 mm OD .times.7.6 cm
length). The temperature of the reaction is measured from a stainless
steel thermowell (1/8 inch OD) embedded in the catalyst sample. A
feedstream containing butane (10-20 volume percent) and helium (90-80
volume percent) is passed over the catalyst for about 5-10 seconds. The
flow of the feedstream is stopped and a purge stream comprising pure
helium is passed over the catalyst at the same flow rate for 1 minute. The
purge stream is stopped and a stream of oxygen (20 volume percent) in
helium i3 passed over the catalyst at the same flow rate for 1 minute,
followed by another purge stream of helium for 1 minute. This cycle is
repeated and the combined products are collected in a Saran.RTM.
polyvinylidene chloride plastic bag for analysis. Analysis is performed on
a Carle gas chromatograph designed to analyze for C.sub.1 -C.sub.5
alkanes, alkenes and alkadienes, as well as permanent gases such as
N.sub.2, O.sub.2, CO, CO.sub.2, H.sub.2, and heavier products including
furan, benzene, and C.sub.6 compounds. Isobutane is mixed with the feed or
products as a standard. "Unknowns" are obtained from the difference
between the carbon balance and 100 percent. The process conditions and
results are set forth in Table I.
TABLE I.sup. .circle.1
______________________________________
Example 2 3 4
______________________________________
Wt. Catalyst, g
9.79 10.26 8.96
Wt. % SiO.sub.2
40.00 34.00 40.00
GHSV, hr.sup.-1
1060 994 994
Pulse, sec 5.0 10 10
% Conversion 54.75 40.07 22.52
% Selectivities:
1-butene 3.46 4.77 8.64
tr-2-butene 3.11 3.95 9.09
cis-2-butene 2.53 4.28 7.87
butadiene 71.72 71.53 65.85
Sum C.sub.4 's
80.82 84.53 91.45
propylene 0.00 0.27 1.23
ethylene 0.00 1.78 2.14
% Total Cracking
0.00 2.05 3.36
CO.sub.2 11.88 9.00 4.43
CO 4.42 3.64 0.77
% Deep Oxidation
16.30 12.64 5.19
furan/benzene 0.93 0.78 0.00
Unknown 1.97 0.00 0.00
% Total Heavies
2.89 0.78 0.00
Total C balance
98.92 100.09 101.7
g C.sub.4 /g cat-hr
0.30 0.23 0.16
g C.sub.4 /g cat-hr.sup. .circle.2
0.26 0.20 0.12
% Yield C.sub.4 's
44.25 33.87 20.60
______________________________________
.sup. .circle.1 Butane, 20 vol. %; Rxn. temperature, 580.degree. C.
.sup. .circle.2 BD is butadiene.
It is seen that the catalyst composition containing the above-described
silica matrix and oxides of magnesium and molybdenum i3 highly active and
selective in the oxidation of butane to butenes and butadiene (BD).
EXAMPLE 3 --CATALYST PREPARATION AND BUTANE OXIDATION
A catalyst composition is prepared as in Example 1, with the exception that
magnesium oxide (90 g) is used instead of magnesium hydroxide during PVA
treatment and PVA-treated magnesium oxide powder (34 g) is added to the
potassium silicate solution. The composition thus prepared is essentially
identical to the composition of Example 1. Moreover, the catalyst
composition prepared with magnesium oxide exhibits a crush strength
comparable to the crush strength of the catalyst composition in Example 1
and is therefore suitable for use in a riser reactor. The catalyst
prepared with magnesium oxide is tested in the oxidation of butane
according to the procedure of Example 2 with the results set forth in
Table I. It is seen that the catalyst is highly selective and active in
the oxidation of butane to butenes and butadiene.
EXAMPLE 4 --CATALYST PREPARATION AND BUTANE OXIDATION
Magnesium oxide (60 g) is added with mixing to a solution containing water
(120 g) and 21 weight percent polyacrylic acid (50 g; 90,000 MW). The
mixture is dried in a nitrogen-purged oven at 80.degree. C. for 18 hr. The
resulting polyacrylic acid-treated magnesium oxide is rough crushed,
heated further at 125.degree. C. for 4 hr, and crushed again to pass a 170
mesh screen (88 .mu.m). The solid obtained is blended into a potassium
silicate solution which is polymerized as in Example 1. The resulting
composite is washed with ammonium nitrate, impregnated with a solution of
ammonium heptamolybdate and calcined, per Example 1, to yield a catalyst
composition of adequate hardness for use in a riser reactor. The catalyst
composition is essentially identical to that of Example 1 and contains the
above-identified silica matrix and domains of a catalyst component
comprising magnesium oxide and molybdenum oxide.
The above-identified catalyst is tested in the oxidation of butane
according to the procedure of Example 2 with the results set forth in
Table I. It is seen that the catalyst composition prepared with a blocking
agent of polyacrylic acid instead of poly(vinyl alcohol) is also highly
active and selective in the oxidation of butane to butenes and butadiene
(BD).
EXAMPLE 5 --CATALYST PREPARATION AND BUTANE OXIDATION
A catalyst composition is prepared as in Example 1 with the exception that
the slurry containing poly(vinyl alcohol)-treated magnesium hydroxide,
formamide and potassium silicate is suspension polymerized into spheroidal
particles rather than polymerized in batch. The suspension polymerization
method involves adding the slurry slowly at 10.degree.-12.degree. C. to
o-dichlorobenzene (The Dow Chemical Company Dowtherm E.RTM.), which
additionally contains 1 percent by weight fumed silica as a dispersion
agent. The mixture is then agitated using a low shear mixer for a period
of time sufficient to break the aqueous phase into droplets. The
temperature is then raised to 80.degree. C. for 1.5 hr during which time
the silicate cures to form spheroidal particles. The particles are washed
with acetone to remove the Dowtherm E.RTM.. Thereafter, the particles are
aged for 18 hr, washed, dried and calcined as per Example 1. Specifically,
the calcination is conducted for 2 hr at 100.degree.-150.degree. C., 4 hr
at 150.degree.-300.degree. C., 1 hr at 300.degree.-400.degree. C., 4 hr at
400.degree.-450.degree. C., 2 hr at 450.degree.-600.degree. C., and 4 hr
at 600.degree.-610.degree. C. The resulting catalyst composition comprises
a silica matrix essentially identical to that described in Example 1.
Encapsulated in the matrix are domains of magnesium oxide containing
molybdenum oxide. The crush strength of the spheroidal particle is 1.34 lb
.+-.0.29 lb, as measured on a particle of about 600 .mu.m. It is seen that
the composition prepared by suspension polymerization is strong enough for
use in a riser reactor.
The catalyst prepared hereinabove is tested in the oxidation of butane
according to the procedure of Example 2 with the results set forth in
Table II. It is seen that the catalyst composition is highly active and
selective in the oxidation of butane to butenes and butadiene (BD).
TABLE II.sup. .circle.1
______________________________________
Example 5 6 7
______________________________________
Wt. Catalyst, g
11.40 11.00 11.00
Wt. % SiO.sub.2
35.0 35.0 35.0
Wt. % K.sup.+, g
0.0 0.1 0.2
GHSV, hr.sup.-1
994 1039 1026
% Conversion 49.03 43.19 35.67
% Selectivities:
1-butene 3.46 4.68 7.02
tr-2-butene 2.77 3.65 4.86
cis-2-butene 2.31 3.76 4.75
butadiene 57.07 64.49 66.69
Sum C.sub.4 's
65.63 76.59 83.33
propylene 1.41 0.00 1.01
ethylene 1.96 2.37 2.78
% Total Cracking
3.38 3.43 3.79
CO.sub.2 18.00 11.14 5.85
CO 8.85 5.54 2.79
% Deep Oxidation
26.85 16.69 8.65
furan/benzene 4.13 2.52 1.97
Unknown 0.00 0.76 2.25
% Total Heavies
4.13 3.28 4.22
Total C balance
100.53 99.67 99.19
g C.sub.4 /g cat-hr
0.20 0.21 0.19
g BD/g cat-hr.sup. .circle.2
0.17 0.17 0.15
% Yield C.sub.4 's
32.18 33.08 29.72
______________________________________
.sup. .circle.1 Butane, 20 vol. %; Rxn. T, 580.degree. C.; 10 sec pulse.
.sup. .circle.2 BD is butadiene.
EXAMPLE 6 --CATALYST PREPARATION AND BUTANE OXIDATION
A catalyst composition (11.0 g) prepared as in Example 5 is impregnated
with a solution comprising methanol (5.84 g) and potassium hydroxide
(0.018 g). The impregnated catalyst is dried and calcined as in Example 5
to yield a catalyst composition having a potassium concentration of 0.1
weight percent. The crush strength of the catalyst gives a maximum load of
1.34 lb .+-.0.29 for spheroidal particles of 600 .mu.m size. It is seen
that the strength of the potassium-doped catalyst is sufficient for use in
a transport reactor.
The catalyst is tested in the oxidation of butane according to the method
of Example 2 with the results set forth in Table II. It is seen that the
potassium-promoted catalyst composition achieves high selectivity and
productivity for butenes and butadiene. When Example 6 is compared with
Example 5 it is seen that the catalyst composition containing potassium
achieves a significantly higher selectivity to C.sub.4 olefins with only a
slight reduction in conversion.
EXAMPLE 7 --CATALYST PREPARATION AND BUTANE OXIDATION
A catalyst composition prepared and impregnated with potassium as in
Example 6 is impregnated again with a solution comprising methanol (5.84
g) and potassium hydroxide (0.018 g). The impregnated composition is dried
overnight and calcined as in Example 5 to yield a composition containing
0.2 weight percent potassium. The crush strength of the spheroidal
catalyst particles of 600 .mu.m size is 1.34.+-.0.29, therefore the
composition is suitable for use in a riser reactor. The catalyst
composition is employed in the oxidation of butane with the results shown
in Table II. It is seen that the potassium-promoted catalyst composition
achieves high selectivity and productivity for butenes and butadiene.
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